The project will be centered on a genetic and biochemical analysis of the regulation of the branched-chain amino acids in yeast, S. typhimurium and E. coli. Of particular interest will be the precise chromosomal location of mutations affecting the gene expression in the ilv gene cluster and to define precisely those mutations in the ilvA gene that affect threonine deaminase activity and those that affect instead, or in addition, the function of the ilvB, ilvC gene products or affect expression of the ilvADE operon. Another goal will be to continue the mesurement in E. coli of ilv mRNA corresponding to the repressible genes ADE and B and to the inducible gene ilvC and to determine the kinetic behavior of message formation and breakdown in ilv regulatory mutants. Attempts to transcribe ilv mRNA in vitro will be made in order to define biochemically the role of the presumed regulatory elements.